It's not rocket science.
Get some basic stuff down so that it's really, really YOURS. You can say it correctly and understand it immediately when others say it. On that foundation, your (then smiling, hopefully) in-laws will help you learn more -- once you repeatedly drill it into their heads that they must slow down and sometimes write things down for you. You'll start hearing things that were there all the time, only you didn't realize it because you didn't understand that little piece of the language.
A little Mandarin ability in this sort of situation (if the family is not a bunch of stiffs in the first place) goes a long ways to producing goodwill, which then multiplies and reinforces further proficiency. But what you need is ability, not memorized stuff. If memorized stuff worked, every student who started a class on the island would become fluent.







